I agree they most likely were Islamist, although incidentally saying Allahu Akbar is not a very precise way of identifying an Islamist:
https://en.wikipedia.org/wiki/TakbirTo quote from the page:
'In videos released during the course of the Syrian Civil War, Free Syrian Army, Al-Nusra Front, other rebel and Islamist groups and ISIL are heard shouting "Takbir" and "Allahu Akbar"'
Some of those are Islamist groups, some are not.
Also, we don't know if the perpetrators were French or not, so what 42% of young French Muslims believe is not relevant at this point. Even if it was relevant, it would be hard to draw any conclusions from it. In the same link you provide, it seems that over 75% of French Muslims are concerned about the rise of Islamic Extremism.